Centrifugal dryers and their use in the pharmaceutical and chemical industries are well known in the art. These dryers combine the two steps of centrifuging and drying into one apparatus. For example, in the isolation of pharmaceutical active substances, there are applications which require a combination of centrifugation at one time in a single unit and in which, by means of the centrifugal dryer, unintentional release during transport from the centrifuge to the dryer is avoided. Other applications thereof include the isolation of oxidation-sensitive or light-sensitive products as well as drying/moisture removal applications in aseptic processes.
U.S. Pat. No. 6,408,536 to Deusser et. al. discloses and claims a method for drying protein crystals starting from an aqueous protein crystal suspension in a centrifugal dryer. For example, insulin is used for the preparation of pharmaceutical formulations in crystalline form. For the stability of the crystal structure, insulin crystals require an optimum residual water content of from 2% to 10%. The drying process must therefore be appropriately and precisely stopped when this optimum residual water content range is reached.
The control of the drying process in the centrifugal dryer was achieved by known dryers of the prior art in two ways. First, a minimum drying time is established as a function of the loading of the dryer, and secondly, the drying process is stopped before the moisture in the N2 drying exhaust gas falls below the 2.0% limit. For determining the actual residual moisture in the dried product, a sample of the product is then taken and the water content of the sample is determined by means of a Karl Fischer titration. However, the procedure in this method is relatively complicated despite the use of semi-automatic devices. The size of the sample quantity required for such a determination is within about a few hundred mg. The time required for a titration is typically from 5 to 20 minutes. The disadvantages of the prior art methods are that the samples have to be removed from the process whereby the method may detrimentally effect the chemical/pharmaceutical so dried and this also requires a substantial period of time. Moreover, particular chemicals that can be dried in this manner and a specific extraction are required and these can also be dried in the methods and dryers of the prior art.
Published U.S. Patent Appln. No. 2005000869 to Kessler et. al. discloses and claims a device for determining the residual liquid content of solid cakes in centrifuges, consisting of at least two sensors, in particular electrodes, and a measuring device connected to the sensors and intended for measuring the residual moisture, in particular a conductivity-measuring or capacitance-measuring device, the sensors being arranged a distance apart in the region of the centrifuge drum. However, this device has the disadvantage that there is direct contact between the product and the sensors so that deposits of the product on said sensors may occur. Mounting the sensors and measuring device on the rotating drum furthermore results in complicated arrangements for data transmission and for the voltage supply of the device, and corrosion protection precautions have to be taken.
DE A1 196 45 923 assigned to Bayer AG discloses another device for an on-line moisture measurement that is in contact with the product in a stationary (non-rotating) dryer. An optical measuring head for an on-line investigation of the moisture of agglomerated particles in dryers is used in, which measuring head consists of a pot-like sample chamber arranged vertically in the dryer and open at the top and an optical sensor which is closed in the sample chamber with a measuring window and produces an output signal characteristic of light diffusely reflected by the particle sample in the sample chamber. However, this measuring head is not rotatably moveable and is not suitable for a rotating centrifuge drum or centrifugal dryer.
Finally, U.S. Pat. Nos. 6,063,292 and 6,328,897 to Leung relates to a further method in which the surface cake moisture is measured in situ using an infrared reflection from a cake of solids within the drum of a drum centrifuge. A cake moisture monitoring means is arranged in the drum. A disadvantage of this method is once again the contact between product and cake moisture monitoring means. Furthermore, the surface moisture of the cake and not the average moisture content of the solids can be determined.
The object of the present invention avoids the disadvantages of the prior art and in particular, permits the on-line determination of the residual moisture of a material present in a dryer comprising a moving drying compartment, in a centrifugal dryer, without the need for contacting the product or removal thereof from the dryer.